Process for purifying silver compounds

ABSTRACT

Heavy metal impurities may be separated from silver ion by precipitating and separating silver acetylide from an impure silver salt solution. Reaction of the appropriate acid with the separated silver acetylide may be employed to generate a purified silver salt therefrom. The process of the present invention is useful for purifying aqueous silver nitrate solutions for use in manufacturing photographic silver halide emulsions.

United States Patent [1 1 Long et al.

[ Mar. 26, 1974 PROCESS FOR PURIFYING SILVER COMPOUNDS [75] Inventors:Darla Long, Boston; Kenneth Norland, Belmont, both of Mass.

' [73] Assignee: Polaroid Corporation, Cambridge,

Mass.

221 Filed: Man, 1972 [21] Appl. No.: 249,513

[56] References Cited UNITED STATES PATENTS 8/1945 Hoff 423/395 X 1/1969Long 260/438.1

OTHER PUBLICATIONS McPherson & Henderson Book, A Course in GeneralChemistry, Third Ed., 1927, pages 272 and 273,

Ginn and Co., New York, Personal Copy in Grays A.U. 113.

An Outline of Organic Chemistry," College Outline Series, by Degering etal., (1937 Ed.), pages 33 and 34, Barnes & Noble, lnc., New York.

Organic Chemistry An Outline, by C. Hansch and G. l-lelmkamp, p. 17,1959 Ed., McGraw-Hill Book Co., Inc., New York.

The Chem. Elements and Their Compounds, 1950, pages 112 & 113, N.V.Sidgwick, Vol. 1, Oxford.

Primary Examiner-Edward Stern [5 7] ABSTRACT Heavy metal impurities maybe separated from silver ion by precipitating and separating silveracetylide from an impure silver salt solution. Reaction of theappropriate acid with the separated silver acetylide may be employed togenerate a purified silver salt therefrom. The process of the presentinvention is useful for purifying aqueous silver nitrate solutions foruse in manufacturing photographic silver halide emulsions.

10 Claims, No Drawings 1 PROCESS FOR PURIFYING SILVER COMPOUNDSBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to chemistry and particularly to the purification of silvercompounds.

2. Description of the Prior Art The most common impurities present insilver compounds are the heavy metals, i.e., those metals occupying thelower half of the Periodic Table of Elements. Since lead, lead-zinc,copper, gold and copper-nickel ores are the major sources of silver, itcan be appreciated that the elimination of these heavy metals is ofgeneral concern in connection with the purification of all silvercompounds. Of course, however, other heavy metal impurities may be ofmore particular importance depending on the silver compound involved andits ultimate use.

Silver nitrate is a particularly useful silver compound and findsextensive employment in the field ofphotography as the source of silverions in the manufacture of halide emulsion, and generally increase thefog level..

Likewise, nickel impurities are known to reduce sensitivity andsometimes increase the contrast. Rhodium, iridium and palladiumimpurities have been found to have particularly powerful action onsilver halide emulsions, and may result ina reduction in sensitivityeven when present in the parts per billion range. See: Glafkedis, P.,Photographic Chemistry; Foundation Press; London (1960) Pg. 318.

Various procedures for the elimination of the afore-. mentioned andother heavy metal impurities from aqueous silver nitrate solutions havebeen described in the art. Forexample, a common method consists of thefractional recrystallization of the silver nitrate from a nitric acidsolution a time-consuming-and tedious procedure. Another method involvesprecipitating silver by means of copper and redissolving the silver innitric acid; however, copper is expensive to use and thephotographically harmful metal palladium, if present, is not removed.See: Sneed, Maynard and Brasteel; Comprehensive Inorganic Chemistry,Vol. II, 1954) D. Von Nostrand Co., Inc.; New York; Pg. 165.

US. Pat. No. 2,543,792 describes a process for purifying aqueous silvernitrate solutions to photographic quality by passing the silver nitratesolution into contact with elemental carbon, passing the resultingsolution into contact with activated alumina and subsequently filteringthe solution through a silver oxide impregnated filter. This process,however, has the disadvantage that alumina must be replaced orregenerated frequently because it is exhausted rapidly by the acidnature of the silver nitrate solution and the relatively large amountsof metal impurities which must be removed.

Other methods in the art generally involve the addition of silver oxideto the aqueous silver nitrate solution in an amount sufficient to raisethe pH to a specified level, thereby precipitating the metallicimpurities. See, for example, US. Pat. No. 3,141,731 (addition of silveroxide and iron nitrate with heating); US. Pat. No. 2,614,029 (additionof silver oxide to a pH of 6.1-9.0 followed by treatment of the solutionwith absorbentssuch as alumina, magnesia and elemental carbon) andFrench Pat. No. 2,010,282 (.addition of silver oxide to a pH of 5 l5.8;followed by another addition to a pH of 5.9-6.3).

BRIEF SUMMARY OF THE INVENTION It has now been discovered that heavymetal ions particularly those having a valence greater than one, may beeffectively separated from the silver ion of a soluble silver salt,e.g., silver nitrate, by reacting an acetylene with a silver saltsolution containing other heavy metal ions to form a precipitate ofsilver acetylide which is subsequently separated from the supernatantliquid. The heavy metal ion impurities are retained in solution and arethereby separated from the silver ion, which has been removed as silveracetylide. If desired, the silver acetylide may then be reacted with anacid to generate the silver salt of the acid used.

Preferably, the above-described silver acetylide precipitation takesplace by passing an acetylene gas, preferably methyl acetylene gas, intoan aqueous solution of, for example, silver nitrate having a silver ionconcentration less than 1.0 M, preferably about 0.05 M, and buffered toa pH less than 7 with an aqueous acetic acid-sodium acetate buffer.

The purification process of the present invention may find particularutility in preparing very pure silver nitrate which may in turn beemployed, for example, to manufacture relatively impurity-freephotosensitive silver halide emulsions or may be used in otherapplications calling for silver ion free from heavy metal ionimpurities.

An object of the present invention is therefore to provide'a simple andeffective process for separating polyvalent heavy metal ions frommonovalent silver ion.

' Another object is to providea process for purifying silver. saltsolutions. 1

Another object is to provide a process for purifying aqueous silvernitrate solutions for use in photography.-

A further object is to provide a process for preparing photosensitivesilver halide emulsions having a reduced amount of heavy metal ionimpurities.

Other objects of this invention will in part be obvious and in partappear hereinafter.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS Apreferable extension of this procedure takes advantage of the propertiesof the separated silver acetylide and involves:

3. reacting the separated silver acetylide with the appropriate acid toliberate acetylene and also generate a purified silver salt.

The term silver salt as used herein is intended to encompass any of thenumerous compounds formed by the replacement of part or all of the acidhydrogen of an acid by silver ion, which compound will react with anacetylene gas to form silver acetylide. The term 7 soluble designates asolubility which is greater than that of silver acetylide in aparticular medium. The terms acetylene and silver acetylide are intendedherein and in the appended claims to be generic terms designatingacetylene, C l-l and silver acetylide, Ag C per se as well assubstituted derivatives thereof. The substituted acetylenes contemplatedas suitable for the practice of this invention may be represented by theformula wherein R is a group such as alkyl, aryl, etc.; preferably, R isa methyl group. Of course, it will be readily apparent to those skilledin the art that R does not enter into the reaction forming theacetylide, and therefore may be any group which does not interfere withthe formation, precipitation or separation of silver acetylide accordingto the processes herein described.

The preferred embodiment of the present invention is specificallydirected to the purification of silver nitrate solutionsplmpuresolutions of silver nitrate of the type purifiable by the process ofthis invention may be solutions derived from the dissolution ofcommercially available silver nitrate in a suitable medium or from thedissolution of silver ingots in nitric acid or even from the electrolyteresulting from the refining of silver by electrolysis. However derived,the above-described silver nitrate solutions contain a certain amount ofundesirable heavy metal impurities such as, for example, copper, lead,nickel, gold, iron, tin, zinc, chromium, bismuth, magnesium, cadmium,palladium, mercury, rhodium, iridium, manganese, aluminum, platinum,rubidium, antimony, etc.

As examples of dissolution media for silver nitrate mention may be madeof any of those which allow the effective precipitation of silveracetylide therefrom, such as, for example, alcoholic or aqueous media.Particularly advantageous results have been obtained with aqueous silvernitrate solutions, for example, the solutions employed in thephotographic art for preparing silver halides.

The first step of the purification of aqueous silver nitrate solutionsaccording to the preferred embodiment of the present invention involvesthe reaction of a quantity of an acetylene gas, e.g., C H or morepreferably, methyl acetylene, CH C H, with the silver nitrate solutionto form a precipitate of a silver acetylide, e.g., Ag C or AgC Cl-lrespectively. The above-described step may be effected by slowlybubbling the acetylene gas through the silver nitrate solution at roomtemperature. The silver acetylide thus formed precipitates from solutionas a solid, and the bubbling of the gas is preferably continued untilthe solution has cleared.

It has been found that improved results are obtained in theprecipitation step just described if the concentration of silver ion inthe impure solution to be purified is less than 1.0 M, and mostadvantageous results may be obtained at a silver concentration of about0.05 M. Silver concentrations much in excess of 1.0 M have been found tobe undesirable because of the handling difficulties encountered inseparating the voluminous precipitate produced at such highconcentrations.

Since the reaction forming silver acetylide involves the generation of acertain amount of acid and a resultant decrease in solution pH, it hasalso been found advantageous to provide a buffer in the silver saltsolution in order to maintain a reasonably constant pH during theprecipitation. Maintaining a pH value below about 7 is advisable in thepractice of this aspect of the invention since some undesirableprecipitation of silver hydroxide and of the heavy metal ion impuritiesmay occur at higher pH values. The maintenance of the desired pHinterval (e.g., from 4 to 6) may be accomplished by employing anyappropriate buffer, preferably a dilute aqueous buffer solution ofsodium acetate.

Any effective process for the separation of a solid from a liquid, orany combination thereof, may be used for the separation of the silveracetylide precipitate formed in the above-described first step of theprocess such as, for example, by centrifugation, filtration,sedimentation and/or decantation. Once removed from the mother liquor,the silver acetylide may then undergo a washing procedure effected toremove any adsorbed impurities remaining thereon, if desired.

It should be noted that silver acetylide, Ag C is highly explosive ifallowed to dry, and detonates even on gentle friction. For this reason,it is much more preferable to employ the precipitation of non-explosiveacetylides such as, for example, silver methyl acetylide, AgC Cl-l inthe practice of the herein-described processes. If for any reason theprecipitation of Ag C is employed, the silver acetylide should not bedried or allowed to stand exposed to heat for any length of time.Furthermore, it is advisable to employ laboratory or commercialprocedures wherein any separated silver acetylide is immediatelysubjected to the third step explained hereinafter, without allowing anyappreciable amount of the separated material to become dry.

It has long been known that acetylene has the property of reacting withmany heavy metals to form. insolu-. ble acetylidesThese heavy metalsgenerally occupy Groups 18 and HE of the Periodic Table, and includesilver, copper, zinc and cadmium.

It was therefore surprising to find that the selective separation ofsilver ion as silver acetylide according to this invention is possiblefrom a solution which also contains the heavy metal ions which wouldnormally be expected to likewise form acetylides which coprecipitatefrom solution. It has been hypothesized that this selective separationmay be a function of the relative ease in which the monovalent silverion can enter into the acetylide crystal lattice over the higher valenceheavy metal ions commonly found as impurities in silver compounds. Atany rate, due to the expected adsorption of a relatively large amount ofthe heavy metal impurities to the surfaces of the silver acetylidecrystals, one would not even anticipate any degree of success for aselective separation of silver ion from those heavy metals which do notform acetylides.

However, contrary to the above-mentioned expectations, processes areherein disclosed whereby a silver acetylide may be precipitated andseparated from other heavy metal ions, particularly those havingvalences greater than 1, to provide silver ion with vastly reduced.levels of contamination from such ions, for example,

levels below one part per million. The discovery that a selectiveseparation of silver ion from ions of other heavy metals is effectedthrough the precipitation of silver acetylide forms the substance of thepresent invention.

Separation of the silver ion as a silver acetylide is particularlyadvantageous when the desired end product is a purified silver salt, forexample, purified silver nitrate. By utilizing the well-known propertyof acetylides to react with an acid and liberate acetylene gas, a silversalt of the acid employed may be generated from the separated silveracetylide. The acid employed for this reaction should be substantiallyfree of heavy metal ions and other impurities so that a silver salt maybe obtained which is of considerably higher purity than the silver saltwhich was initially reacted with the acetylene to form the silveracetylide.

Thus, in the practice of one aspect of this invention, high purity acid,for example, nitric acid, may be reacted with the silver acetylide, saidacetylide having been formed and purified as a result of the first twosteps described hereinbefore, to provide a silver salt solution of veryhigh purity. Extremely pure acids suitable for employment as describedimmediately above are commercially available, e.g., the acid marketedunder the Ultrex trademark of JVT. Baker.

It should be recognized andobvious to those in the art that one is notnecessarily limited to regenerating the silver salt initially employedto form the silver acetylide, but mayform any desired silver saltdepending on the choice of acid employed. Thus, the generation of anysuch silver salt or elemental silver itself from a silver acetylidewhich has been separated from a silver salt solution for the purposes ofpurification is intended to fallwithin the scope of the presentinvention.

Aqueous silver nitrate solutions purified by the process forming thesubject matter of the present invention may be advantageously employedin avariety of ways, for example, in the manufacture of photographicsilver halide emulsions. As examples of photographically harmful heavymetal ions which may be separated from silver ion by the practice ofthis invention, mention may bemade of the polyvalent ions of Mn, Ni, Co,Fe, Rh, Pb, Cd, Zn, Cu, Ir, Pd and Pt.

In a typical photographic emulsion preparation embodying this invention,a silver nitrate solution purified by the above-described novelthree-step process may be reacted with at least one water-solublehalide, such as ammonium, potassium or sodium bromide, preferablytogether with a corresponding iodide, in an aqueous solution of aprotective colloid such as a colloidal gelatin solution according to thetraditional procedures of the art, as described, for example, inNeblette, C. B.; Photography, Its Materials and Processes, 6th Ed.,1962.

Alternatively, it is contemplated that the precipitation of silveracetylide according to this invention may be employed to provide asource of high purity silver ion for reaction with a high purity halogengas, e.g., bromine, to produce silver halide essentially in accordancewith known procedures in the art such as, for example, the process ofMalinowski as described in The Journal of Photographic Science, vol. 8,1960, pages 69-71. Such silver halide may then be employed for thefabrication of photographic elements, including those formed byevaporation of silver halide and the condensation of the vapors on asubstrate surface to form thereon a stratum of light sensitive silverhalide suitable for photographically recording images. For a generaldiscussion of processes of the foregoing type see: U.S. Pat. No.3,297,463.

The invention will be further illustrated in conjunction with thefollowing specific examples which are intended to be illustrative andnot to be interpreted as limiting in any way.

Example I About cc of a 0.06 M aqueous silver nitrate solution(equivalent to about 1 gm of AgNO was mixed in a suitable vessel withabout 7 cc. of a 1.0 M aqueous sodium acetate solution to form abuffered AgNO solution having a pH in the range of from 5 to 6. Thisbuffered AgNO solution was then contaminated" with heavy metal ionimpurities by doping" the AgNO solution with a solution comprising thenitrates of the metal ions Mnf Ni, Co, Fe, Rh, Pb, Cd, Zn and Cu; eachion at a concentration of about 20 ugm/gm of AgNO in solution. In thismanner, a silver nitrate solution was provided having a known heavymetal ion impurity level in the vicinity of about 20 parts per millionfor each contaminating ion.

Next, methyl acetylene gas was bubbled through the doped" silver nitratesolution prepared above. A white precipitate of silver-methyl acetylideformed immediately and the bubbling of the gas was continued until theprecipitation had been completed (as evidenced by solution clarity and asolution pH of about 4). The precipitate was then removed by filtrationand the remaining supernatant analyzed for the content of each of theabove mentioned ions by employing a Perkin Elmer Model 403 AtomicAbsorption Spectrophotometer.

Example ll A buffered AgNO solution was prepared exactly as described inExample I. However, this AgNO solution was not doped before the methylacetylene gas was bubbled therethrough. The precipitate formed as aresult of the reaction with methyl acetylene was first removed byfiltration and then the doping solution described in Example I was addedto the remaining supernatant, which was subsequently analyzed by atomicabsorption spectrophotometry as in Example I.

Table A below summarizes the results obtained when the analysis for eachion remaining in the supernatant of Example I was compared with theanalysis for the same ion in the supernatant of Example II. The resultsare expressed as percentages of a particular ion that were foundremaining in the supernatant of Example I, making the assumption thatthe amount of that ion found in the supernatant of Example ll representswhat was initially present in Example I:

Table A Percentage remaining in Example I supernatant Table A-ContinuedPercentage remaining in Example I supernatant Metallic [on (andtherefore separated from Ag' Rh- 96% Pb 96% Cd 95% Zn 99% Cu 60% It canbe seen from Table A that a substantial amount I of each of the heavymetal ions remain in solution,

thereby separating these impurities from the silver ion which wasprecipitated and separated as silver methyl acetylide according to thepresent invention.

Examples Ill and IV Table B Metallic lon. Percentage remaining inExample I supernatant Mn 120% Ni 92% Co 96% Rh 55% Pb 90% Cd 85% Zn"100% Cu 70% Again, it can be seen from Table B that at least a majorityof every heavy metal ion remains in the supematant of Example lll afterthe precipitation of silver acetylide therefrom.

Since certain changes may be made in the above processes withoutdeparting from the scope of the invention herein described, it isintended that all matter contained in the above description shall beinterpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A process of purifying a soluble silver salt which comprises thesteps of;

dissolving the impure soluble silver salt to form a solution containingmonovalent silver ion and at least one contaminating heavy metal ionhaving a valence greater than one;

reacting an acetylene with said solution to form a precipitate of silveracetylide, at least a majority of said contaminating heavy metal ionremaining in solution;

separating said silver acetylide precipitate from the supernatant liquidcontaining said majority of contaminating heavy metal ion; and

reacting said separated silver acetylide with an acid to generate a highpurity silver salt of said acid.

2. A process as defined in claim 1 wherein said acetylene is methylacetylene and said silver acetylide is silver methyl acetylide.

3. A process as defined in claim 1 wherein said silver salt is silvernitrate and said acid is nitric acid.

4. A process as defined in claim 1 wherein said solution is aqueous.

5. A process as'defined in claim 4 wherein the concentration ofmonovalent silver ion in said aqueous solution is less than 1.0 M-andthe solution pH is less than 7.

6. A process as defined in claim 1 wherein said contaminating heavymetal ion is a polyvalent ion of Mn, Ni, Co, Fe, Rh, Pb, Cd, Zn, Cu, Ir,Pd or Pt.

7. A process for preparing high purity photosensitive silver halidewhich comprises:

dissolving impure silver nitrate in water to form an aqueous solutioncontaining monovalent silver ion and at least one contaminating heavymetal ion having a valence greater than one;

reacting an acetylene gas with said solution to form a precipitate ofsilver acetylide, at least a majority of said contaminating heavy metalion remaining in solution;

separating said silver acetylide precipitate from the supernatant liquidcontaining said majority of contaminating heavy metal ion;

reacting said separated silver acetylide with nitric acid to generatehigh purity silver nitrate; and reacting said high purity silver nitratewith a watersoluble halide to form high purity silver halide.

8. A process as defined in claim 7 wherein said acetylene gas is methylacetylene and said silver acetylide is silver methyl acetylide.

9. A process as defined in claim 7 wherein the concentration of themonovalent silver ion is about 0.05 M and the solution pH is less than7.

10. A process as defined in claim 7 wherein said contaminating heavymetal ion is a polyvalent ion of Mn, Ni, Co, Fe, Rh, Pb, Cd, Zn, Cu, Ir,Pd or Ft.

2. A process as defined in claim 1 wherein said acetylene is methylacetylene and said silver acetylide is silver methyl acetylide.
 3. Aprocess as defined in claim 1 wherein said silver salt is silver nitrateand said acid is nitric acid.
 4. A process as defined in claim 1 whereinsaid solution is aqueous.
 5. A process as defined in claim 4 wherein theconcentration of monovalent silver ion in said aqueous solution is lessthan 1.0 M and the solution pH is less than
 7. 6. A process as definedin claim 1 wherein said contaminating heavy metal ion is a polyvalention of Mn, Ni, Co, Fe, Rh, Pb, Cd, Zn, Cu, Ir, Pd or Pt.
 7. A processfor preparing high purity photosensitive silver halide which comprises:dissolving impure silver nitrate in water to form an aqueous solutioncontaining monovalent silver ion and at least one contaminating heavymetal ion having a valence greater than one; reacting an acetylene gaswith said solution to form a precipitate of silver acetylide, at least amajority of said contaminating heavy metal ion remaining in solution;separating said silver acetylide precipitate from the supernatant liquidcontaining said majority of contaminating heavy metal ion; reacting saidseparated silver acetylide with nitric acid to generate high puritysilver nitrate; and reacting said high purity silver nitrate with awater-soluble halide to form high purity silver halide.
 8. A process asdefined in claim 7 wherein said acetylene gas is methyl acetylene andsaid silver acetylide is silver methyl acetylide.
 9. A process asdefined in claim 7 wherein the concentration of the monovalent silverion is about 0.05 M and the solution pH is less than
 7. 10. A process asdefined in claim 7 wherein said contaminating heavy metal ion is apolyvalent ion of Mn, Ni, Co, Fe, Rh, Pb, Cd, Zn, Cu, Ir, Pd or Pt.